Recent advances in wireless communication technologies have caused repeated evolution of communication systems technologies. As such, network operators have to manage networks in which GSM (second generation mobile communication), UMTS and CDMA2000 (third generation mobile communication), and LTE (fourth generation mobile communication) coexist with each other. This has necessitated mobility support between heterogeneous wireless communication technologies, wireless communication systems, or Radio Access Technologies (RATs).
FIG. 1 illustrates the architecture of the LTE system, to which the present disclosure is applied.
Referring to FIG. 1, the radio access network of the LTE system is composed of Evolved Node Bs (ENB, Node B or base station) 105, 110, 115 and 120, Mobility Management Entity (MME) 125, and Serving Gateway (S-GW) 130. A user equipment (UE or terminal) 135 may connect to an external network through the ENBs 105 to 120 and the S-GW 130.
In FIG. 1, the ENBs 105 to 120 correspond to Node Bs of the existing UMTS system. The ENB is connected to the UE 135 through a radio channel, and may perform more complex functions in comparison to the existing Node B. In the LTE system, as all user traffic including real-time services like Voice over IP (VoIP) services is served by shared channels, an entity is needed to perform scheduling on the basis of status information collected from UEs such as information on buffer states, available transmit power and channels. Each of the ENBs 105 to 120 performs this scheduling function. In most cases, a single ENB controls multiple cells. To achieve a data rate of 100 Mbps, the LTE system utilizes Orthogonal Frequency Division Multiplexing (OFDM) in, for example, a 20 MHz bandwidth as radio access technology. Adaptive modulation and coding (AMC) is employed to determine the modulation scheme and channel coding rate according to UE channel states. The S-GW 130 provides data bearers, and creates and releases a data bearer under control of the MME 125. The MME 125 performs various control functions including UE mobility management and is connected to multiple ENBs.
FIG. 2 illustrates a hierarchy of wireless protocols in the LTE system, to which the present disclosure is applied.
Referring to FIG. 2, for a UE and ENB in the LTE system, the wireless protocol stack is composed of Packet Data Convergence Protocol (PDCP) 205 or 240, Radio Link Control (RLC) 210 or 235, Medium Access Control (MAC) 215 or 230, and a physical layer (PHY) 220 or 225. The PDCP 205 or 240 performs compression and decompression of IP headers. The RLC 210 or 235 reconfigures PDCP PDUs (Protocol Data Unit) to a suitable size. The MAC 215 or 230 is connected to multiple RLC layer entities in the same UE, and multiplexes RLC PDUs into MAC PDUs or demultiplexes MAC PDUs into RLC PDUs. The physical layer 220 or 225 converts higher layer data into OFDM symbols by means of channel coding and modulation and transmits the OFDM symbols through a wireless channel, or converts OFDM symbols received through a wireless channel into higher layer data by means of demodulation and channel decoding and forwards the data to higher layers. For additional error correction, hybrid ARQ (HARQ) is used in the physical layer, and the receiving end sends 1-bit HARQ ACK/NACK information indicating whether a packet transmitted by the transmitting end is received. Downlink HARQ ACK/NACK information as to uplink transmission may be sent through Physical Hybrid-ARQ Indicator Channel (PHICH), and uplink HARQ ACK/NACK information as to downlink transmission may be sent through Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH).
Meanwhile, an LTE UE in the idle mode may perform cell reselection owing to movement or the like. For movement between cells of different (heterogeneous) RATs (e.g. GSM and UMTS), the LTE UE may use a cell selection receive level value (referred to as Srxlev) in a form of received signal strength indication and a cell selection quality value (referred to as Squal) in a form of received signal quality indication according to versions.
Srxlev and Squal are computed by Equation 1 given below.Srxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−PcompensationSqual=Qqualmeas−(Qqualmin+Qqualminoffset)  [Equation 1]
Here, Qrxlevmeas indicates measured received signal strength and Qqualmeas indicates measured received signal quality. Qrxlevmin indicates minimum power required for operation and Qqualmin indicates minimum quality required for operation. Qrxlevminoffset indicates power offset for base stations with a higher priority and Qqualminoffset indicates quality offset for base stations with a higher priority. Pcompensation indicates a correction parameter set for uplink transmission power of the UE.
The Srxlev value is computed based on Reference Signal Received Power (RSRP) in LTE and is computed based on Received Signal Code Power (RSCP) in UMTS. The Squal value is computed based on Reference Signal Received Quality (RSRQ) in LTE and is computed based on the value of Ec/No in UMTS (the received energy per chip (Ec) of the pilot channel divided by the total noise power density (No)).
When multiple RATs (e.g. GSM, UMTS and LTE) coexist with each other as described above, a UE may have to perform inter-RAT cell reselection. Here, as different RATs employ different communication schemes, the UE may have to use different cell reselection criteria. For example, for cell reselection, only Srxlev is used in GSM, CDMA2000, UMTS and the early LTE system (Release 8 or Rel-8). However, Srxlev and Squal may be used in the recent LTE system (from Release 9 or Rel-9).
This is described in more detail below in connection with the LTE system.
At inter-RAT cell reselection based on Srxlev in LTE Rel-8, when SnonServingCell,x of a cell on evaluated frequency is greater than Threshx,high during a time interval TreselectionRAT, cell reselection to the cell is performed. Here, SnonServingCell,x is the Srxlev value of a cell being measured, and the values Threshx,high and TreselectionRAT are transmitted by the LTE base station through preset SIBs or the like. For example, Threshx,high and TreselectionRAT values for UMTS are transmitted through SIB6; Threshx,high and TreselectionRAT values for GSM are transmitted through SIB7; and Threshx,high and TreselectionRAT values for CDMA2000 are transmitted through SIB8.
However, in LTE Rel-9 and later, cell reselection may be performed on the basis of Srxlev and Squal according to circumstances. That is, in LTE Rel-9 and later, when the threshServingLowQ value is provided through SIB3, cell reselection is performed based on Squal. Otherwise, cell reselection is performed based on Srxlev. Specifically, in the event that the threshServingLowQ value is provided through SIB3, cell reselection to a UMTS cell is performed when Squal is greater than ThreshX,HighQ during a time interval TreselectionRAT; and cell reselection to a GSM or CDMA2000 cell is performed when Srxlev is greater than ThreshX,HighP during a time interval TreselectionRAT. In the event that the threshServingLowQ value is not provided through SIB3, cell reselection to a UMTS, GSM or CDMA2000 cell is performed when Srxlev is greater than ThreshX,HighP during a time interval TreselectionRAT. TreselectionRAT, ThreshX,HighQ and ThreshX,HighP values are transmitted through preset SIBs. More specifically, TreselectionRAT, ThreshX,HighQ and ThreshX,HighP values for UMTS are transmitted through SIB6; TreselectionRAT and ThreshX,HighP values for GSM are transmitted through SIB7; and TreselectionRAT and ThreshX,HighP values for CDMA2000 are transmitted through SIB8.
Accordingly, up to LTE Rel-8, inter-RAT cell reselection may be performed solely on the basis of Srxlev. However, when a latest LTE system (LTE Rel-9 or later) coexists with existing 2G and 3G systems, the following problem may arise. That is, a UE may perform inter-RAT cell reselection based on Srxlev while remaining in a GSM, CDMA2000 or UMTS cell, and perform inter-RAT cell reselection based on Squal while remaining in an LTE cell. In this case, the UE in the LTE cell may perform Squal-based cell reselection to a UMTS cell owing to a low Squal value. Immediately thereafter, the UE in the UMTS cell may perform Srxlev-based cell reselection to an LTE cell owing to a low Srxlev value. Such a ping-pong effect should be resolved.
FIG. 3 illustrates a problem that may arise at cell reselection when multiple RATs coexist with each other.
In FIG. 3, the UE 301 capable of supporting LTE, UMTS and GSM communication remains in an LTE base station 303 at time T1. At operation 311, the UE supporting LTE Rel-9 performs Squal-based cell reselection to a UMTS cell. At operation 313, the UE remaining in the UMTS cell performs cell reselection to a GSM cell using a permitted measurement scheme. As the UE supports GSM Rel-6, the UE cannot perform Squal-based cell reselection in the GSM cell. At operation 315, the UE in the GSM cell performs Srxlev-based cell reselection back to the LTE cell. After returning to the LTE base station, at operation 317, the UE performs Squal-based cell reselection to the UMTS cell. At operation 319, the UE in the UMTS cell performs cell reselection to the GSM cell using a permitted measurement scheme. In this way, inter-RAT cell reselection using different schemes in different systems may cause a ping-pong effect.